US2105394A

US2105394A - Manufacture of zinc oxide

Info

Publication number: US2105394A
Application number: US52438A
Authority: US
Inventors: Allavena Riccardo
Original assignee: Pirelli SpA
Current assignee: Pirelli and C SpA
Priority date: 1934-12-01
Filing date: 1935-11-30
Publication date: 1938-01-11
Anticipated expiration: 1955-01-11

Description

`Ian. 1l, 1938. R ALLAVENA 2,105,394

MANUFACTURE oF ZINC @MDE Filed Nov. 50, 1955 Patented Jan. 11, 1938 PATENT OFFICE MANUFACTURE F ZINC OXIDE Riccardo Allavena, Milan, Italy, assignor to Societa Italiana Pirelli, Milan, Italy, a company of Italy Application November 30, 1935, Serial No. 52,438

In Italy December 1, 1934 4 Claims.

This invention is for improvements in or relating tothe manufacture of zinc oxide bythewet process. The improved process comprised by the invention is particularly suitable for the produc-- tion `of zinc oxide in a very finely divided form andof a high degree of activity, especially in' rubber mixings. Y Y .i l AAnurnber of processes are known for obtaining .from zinc ores, zinc oxide to be used either in l,paints orin'rubber mixings. Processes are also known for transforming a zinc oxide having a low'rcovering power into a zinc oinde having a higher covering power. It has been proposed, for examplato dissolve the zinc contained in ores or in waste zinciferous material by means of alkali solutions-so as to obtain an alkali zincate solution, wherefrom a precipitate of zinc hydroxide is afterwards'separated by dilution of the zincate solution with water, if necessary under pressure and at a temperature above normal atmospheric pressure. This process, however, has never come .into practical use as it is expensive and of low Sefliciency; also, it p-resents certain practical diniculties in the iiltration of the precipitates and the `product obtained, being composed mainly of zinc yhydroxides, requires further calcination in order to transform it into zinc oxide. l* According to another known process, zinc hy- :droxide is precipitated from asolution of zinc salts 30 Aby'adding thereto carbonates or bicarbonates or -oxides or hydroxides of alkali metals or alkaline `earth metals. The zinc hydroxide so precipitated is l.rinsed and filtered, after which the ltered precipitate is suspended in'water into which car- 35 vbon dioxide is bubbled for the transformation of fthe suspended hydroxide into zinc carbonate, which, after filtration, is subjected to calcination ata temperature of from 300 C. to 400 C. to obtain Zinc oxide. This process also is expensive "40 and of a low order of efficiency; also it is dimcult to control owing to its complicated character, Vwhich fact tends unfavourably to inuence the 'uniformity of the products. L

"According to the prese-nt invention, a process *15 for the direct manufacture of substantially pure *Zinc oxide, particularly suitable for use in rubber mixings, by precipitation from a solution, consists -intreating an aqueo-us solution of an alkali ff'zincate with a bicarbonate of la metal of which 50 thev normal carbonate is soluble in water, the

bicarbonate being employed, with respect to the -Zincate solution, in substantially stoichiometric proportions for the formation of zinc oxide, 'whereby a'precipitate of substantially pure zinc T55., oxide ispobtained, ltering the said precipitate and drying it at a temperature not exceeding 100 C.

It is known to produce zinc oxide directly from aqueous solutions of alkali zncates by treatment thereof with gaseous carbon dioxide. In carrying 5 this process into practical effect, however, it is found to require a very careful and continuous control of the reaction, owing to variations which tend to occur in the carbon dioxide content of the gases used in this step. Moreover, for this reason 10 .and also owing to the relatively slow activity developed by gaseous carbon dioxide, the reaction tends to be slow, so that the precipitation also proceeds slowly and the zinc oxide particles tend to assume an undesirably large size and to be irregular. By operating in accordance withthe present invention these difficulties are avoided in a simple and eiective manner.

The bicarbonate is preferably used in the form of a solution of alkali bicarbonate produced by 20 bubbling a gas containing carbon dioxide through a solution of an alkali carbonate so as to transform at least part of the carbonate in solution into bicarbonate. It is also possible, however, when economically convenient, to use solid bicarbonate. 25

In carrying the process into elect, it has been found to be advantageous to arrange for a continuous operative cycle in which the alkali hydroxide which is employed in the dissolution of the parent zinc material is regenerated after each 3o cycle from the alkali carbonate which is formed in the bicarbonate treatment of the zincate solution. The accompanying diagrammatic representation illustrates a cyclic process of this character. Thus, the sodium hydroxide which has 3 been transformed into carbonate is regenerated by treatment with milk of lime and utilized again in the cycle together with the carbon dioxide developed in the preparation of thecalcium oxide from the carbonated milk of lime. By thus ar 40 ranging the process, the only substances entering into and issuing from the cycle are substantially the initial zinc ore (which may, if necessary, be

concentrated and free from impurities) and the precipitated zinc oxide end product of the process.

It has already been proposed to treat a solution of zinc salts with bicarbonates, or with carbonates in the presence of free carbon dioxide, with the object of preparing Zinc carbonate. This process, however, is quite different from the present invention, as it employs a solution of zinc salts in place of thezincate solution and it cannot serve for the manufacture of zinc oxide.

The process according to the present invention can be carried out with the employment of zinc V containing' materials of practically any kind, that is with the use of ores, scrap or waste zinciferous material, zinc salts or the like.

The zincate solution may be obtained by direct alkali treatment of the raw material, the latter having previously been ground to a suicient degree of iineness of subdivision, provided the zinc contained in the material be already in the form of oxide; in the contrary event, the raw material will be subjected to calcination to oxidize the zinc. By employing raw material containing oxidized zinc, a good quantitative and qualitative efficiency is available in respect of the process, the extraction of the metal being almost complete, with total separation of iron, cadmium and like extraneous constituents which in the dissolution process remain undissolved. I

In the case of ores containing only small proportions of zinc, it is preferable to concentrate the material before dissolution, in order to prevent considerable loss of alkali and to follow more closely the process cycle illustrated iny the aforesaid diagrammatic representation of the process.

It is also possible, by suitable choice of working conditions, forV instance concentration of the solutions, temperatures, rates of precipitation, etc., to vary within certain limits the properties of the zinc oxide obtained. Inv this manner, it is possible, for example, having at disposal a zinc oxide insufficiently pure or unsuitable for other reasons for certain special applications, to transform and give Value to the zinc oxide by subjecting it to the treatment of the invention.

The process as illustrated in the diagram can be carried into effect as follows;-

grams of calcinated and ground calamine containing 55 per cent. of ZnO is added to each litre of a 20% boiling solution of NaOH. After the mixture has been boiling for several hours a zincate solution containing about r50 grams of zinc oxide per litre is obtained, which is decanted and filtered.

This solution is generally strongly alkaline and therefore, before beginning the bicarbonate treatment of it, the alkalinity of the solution, from the presence of the caustic alkali therein, is eliminated by bubbling carbon dioxide into the solution until precipitation of zinc oxide begins. The calculated amount of bicarbonate is then added rapidly and preferably under hot conditions, the mass being vigorously stirred during the operation. The resulting precipitate of zinc oxide is rinsed and afterwards dried at a temperature not exceeding 100 C.

A solution of sodium carbonate is left after filtration yof the zinc oxide precipitate, which solution, is utilized partly for preparing the bicarbonate required for decomposing the zincate and partly, after causticizing with milk of lime, for dissolving the raw material. Also the carbonate of lime formed in the operation of causticizing the sodium carbonate may be utilized by transforming the same into quick-lime in a suitable kiln, conveying the waste -gases rich in carbon dioxide both to the neutralization of the alkaline solution of zincate, and to the preparation of the bicarbonate solution. The invention, therefore, allows the formation of a cycle of production, wherein the only ingredients entering into the cycle and issuing therefrom are substantially the'r'aw zinciferous material and the precipitated zinc oxide end product of the process.

It is obvious that, according to the raw material to be treated, local conditions and so forth, also for the preventing of excessive dilution of the solutions in course of time, the process as described above may be varied considerably in respect of details without departure from the general principle and scope of the invention, as indicated in the following statement of claim. 5

What I claim and desire to secure by Letters Patent of the United States isz- 1. A process for the direct manufacture of substantially pure zinc oxide by precipitation from a solution, which consists in forming an aqueous 10 solution of an alkali zincate neutralizing the zincate solution to the point of precipitation of zinc oxide, treating the neutralized zincate solution with a bicarbonate of a metal of which the normal carbonate is soluble in water, the bicarl5 bonate being employed, with respect to the zincate solution, in substantially stoichiometric proportions for the formation of zinc oxide, whereby a precipitate of substantially pure zinc oxide is obtained, filtering the said precipitate and drying 20 it at a temperature not exceeding 100 C.

2. A process for the direct manufacture of substantially pure zinc oxide by precipitation from a solution, which consistsin forming an alkaline aqueous solution of an alkali zincate, neutralizing 2`5 the zincate-solution to the point of precipitation of zinc loxide by bubbling gaseous carbon dioxide into it, treating the neutralized zincate solution with a solution of a bicarbonate of a metal of k which the normal carbonate is soluble in Water, V30 the bicarbonate solution being employed, with respect to the zincate solution, in substantially stoichiometric proportions for the formation of zinc oxide, whereby a precipitate of substantially u pure zinc oxide is obtained, ltering the said pre- 35 cipitate and drying it at a temperature not exceeding 100 C.

3. A cyclic process for the direct manufacture of substantially pure zinc oxide by precipitation from a solution, which consists in forming an :40 alkaline aqueous solution of an alkali zincate, neutralizing the zincate solution to the point of precipitation of zinc oxide by bubbling gaseous carbon dioxide into it, treating the neutralized zincate solution with a solution of a bicarbonate t45 of a metal of which the normal carbonate is soluble in water, the bicarbonate solution being employed,with respect to the zincate solution, in substantially stoichiometric proportions for the formation of zinc oxide, whereby a precipitate of 50 substantially pure zinc oxide is obtained with the formation of a solution of alkali carbonate, ltering the said precipitate and drying it at a temperature not exceeding 100 C., regenerating a solution of alkali hydroxide from the alkali car- 5.5 bonate solution by treatment thereof with milk of lime and forming a further quantity of alkali zincate solution from the regenerated alkali hydroxide solution by dissolution of zinc oxide bearing material therein. 'w60 4. A cyclic process for the direct manufacture of substantially pure zinc oxide by precipitation from a solution, which consists in forming an alkaline aqueous solution of an alkali zincate, neutralizing the zincate solution to the point of pre- 1165 cipitation of zinc oxide by bubbling gaseous carbon dioxide into it, treating the neutralized zincate solution with a solution of a bicarbonate of a metal of which the normal carbonate is soluble in water, the bicarbonate solution beingf--70 employed, with respect to the zincate solution, in

`substantially stoichiometric proportions for the formation of zinc oxide, whereby a precipitate of substantially pure zinc oxide is obtained with Ithe formation of a solution of alkali carbonatej'75 filtering the said precipitate and drying it at a temperature not exceeding 100 C., regenerating a solution of alkali hydroxide from the alkali carbonate solution by treatment thereof with milk of lime, forming a further quantity of alkali zincate solution from the regenerated alkali hydroxide solution by dissolution of zinc oxide bearing material therein, the said milk of lime being formed in the process from calcium oxide produced with liberation of carbon dioxide by heating the calcium carbonate which is formed in step of treating with milk of lime the solution of alkali carbonate which is formed in the bicarbonate treatment of the zincate solution, regenerated zincate solution with a part of the carbon 5 dioxide which is liberated in the step of heating the calcium carbonate, the only ingredients entering into and issuing from the cycle being substantially the parent zinc oxide bearing material from which the zincate solution is formed and 10 the substantially pure zinc oxide which is obtained as the product of the process.

RICCARDO ALLAVENA.